A radiation thermometer is for measuring heat radiation intensity (heat emission intensity) radiated from a measuring target object, so as to obtain the temperature of the measuring target object. This radiation thermometer has a characteristic of being able to measure the temperature of the measuring target by a relatively short time without contacting the measuring target object, and thus has a high industrial value. When the temperature of a measuring target object is measured particularly under the circumstances that conditions of temperature, pressure, atmosphere, and the like are changed largely from the external environment, the aforementioned characteristic is exhibited effectively. Moreover, when the measuring target object moves, the characteristic of the radiation thermometer of being a non-contact type is important.
Examples of industrial application using the radiation thermometer include production of semiconductors, production of compound semiconductors containing a nitride system, and the like. In order to produce high-quality semiconductors with high purity, in most cases, the interior of a manufacturing apparatus is isolated from the outside and a substrate retained in the interior of the manufacturing apparatus is heated to a high temperature. In particular, metal organic chemical vapor deposition (MOCVD) for performing film formation on a substrate by subjecting to a chemically active material gas, or molecular beam epitaxy (MBE) for forming a film on a substrate by evaporating constituent elements of a semiconductor in a high vacuum are well known.
For these semiconductor manufacturing apparatuses very precise temperature measurement is required so as to favorably keep uniformity and repeatability of semiconductors produced. As a specific example, the temperature of a measuring target object is in the range of 600° C. to 1200° C., and measurement precision is ±2° C. or less. In practice, in production of a light-emitting element with a multiple quantum well constituted of InGaN (indium gallium nitride) and GaN (gallium nitride) being a light-emitting layer, in the process of producing this light-emitting layer, the substrate is retained at a certain temperature determined from other manufacturing conditions within the range of about 700° C. to 800° C. This certain temperature largely affects the emission wavelength of the light-emitting element, and thus precise temperature measurement as described above is necessary for realizing high emission wavelength uniformity and repeatability.
On the other hand, in order to accurately measure the temperature of a measuring target object by using the radiation thermometer, a value of emissivity of the measuring target object is necessary. As the temperature of an object increases, heat radiation intensity from the object increases, and thus it is possible to measure the temperature of the object by measuring the heat radiation intensity from the object. However, heat radiation intensity from a general object is smaller than heat radiation intensity of a blackbody at the same temperature. The emissivity is obtained by dividing heat radiation intensity from an object at a certain temperature by the intensity of heat radiation from the blackbody at the same temperature. Therefore, by measuring the heat radiation intensity from an object and dividing this heat radiation intensity by the emissivity of this object, the intensity of heat radiation radiated by the blackbody at the same temperature as this object can be obtained, and the temperature of the object can be calculated from this heat radiation intensity. The radiation thermometer using the principle described here can respond to changes of optical parts in various configurations between the radiation thermometer and the measuring target by performing calibration at an appropriate temperature, without performing calibration using the blackbody in a wide temperature range.
Emissivity is measured with various materials, and is published in various documents. In general, many radiation thermometers have a function to store emissivity and use it to correct the intensity of heat radiation from an object, and when the emissivity of the material of a measuring target object is known by a document value or the like, this can be stored for use in the radiation thermometer. However, the emissivity depends not only on the material of the measuring target object but also on the surface condition or temperature. In this sense, the published emissivity is difficult to be used for precise thermometry.
On the other hand, under certain limited conditions, it is possible to measure emissivity. That is, in the wavelength range of light for measuring heat radiation intensity, when the light does not pass through the measuring target object and the light irradiated to the surface of the measuring target object does not scatter, the emissivity (ϵ) is represented by an equation ϵ=1−R, where R is the reflectivity of light on the surface of the measuring target object. Therefore, in the wavelength range of light for measuring heat radiation intensity, when the surface of the measuring target object has sufficient specularity and it is possible to measure the reflectivity of the measuring target object by using an external light source, and the measuring target object absorbs light, the emissivity can be obtained irrespective of the surface condition and temperature of the measuring target object. In order to measure the temperature of the measuring target object with high precision by using such a method, it is crucial to accurately obtain the intensity of heat radiation from the measuring target object and the reflectivity with a preset wavelength.
The radiation thermometer which measures the temperature while obtaining the emissivity of the measuring target object as described above is particularly important for measuring substrate temperature when a thin film is formed on the substrate. In the process of forming the thin film on the substrate, due to occurrence of interference of light by the thin film, the emissivity of the substrate including the thin film changes constantly as the film formation proceeds (the film becomes thick). By this change in emissivity, even when the temperature of the measuring target object is constant, the intensity of heat radiation from the measuring target object changes. Even in such case, when conditions as described above are satisfied, correction of emissivity can be made by measuring the reflectivity appropriately by using the external light source.
However, in order to decrease noise in signals in measurement of heat radiation intensity, it is necessary to widen the wavelength range of light for measuring heat radiation intensity, so as to increase the light intensity to be detected with a detector. For example, when a wavelength band pass filter with a center wavelength of 950 nm and a selected wavelength range of ±25 nm is used (width of the wavelength range=50 nm), signal strength which is five times larger can simply be expected as compared to when the center wavelength is the same and the wavelength range is ±5 nm (width of the wavelength range=10 nm). By obtaining such large signal strength, thermometry becomes possible at low temperatures at which the heat radiation intensity becomes small, allowing lowering the lower limit of the measured temperature. It also becomes possible to obtain sufficient signal strength in a short time, enabling thermometry at higher speed.
On the other hand, mainly two factors are conceivable for the upper limit of the wavelength range for measuring the above-described heat radiation intensity. The first factor is ascribed to that emission energy by heat radiation has wavelength dependence. This wavelength dependence differs depending on the temperature of the measuring target. When the temperature of the blackbody is less than or equal to 1200° C., in the wavelength region of shorter wavelengths than near-infrared, the emission energy becomes smaller as the wavelength becomes shorter. Therefore, when heat radiation is monitored in a certain wavelength range and a temperature is calculated by using the equation of radiation of the blackbody from the measurement values thereof, an error occurs. A specific example is illustrated in FIG. 13. In FIG. 13, for the wavelength range measuring the heat radiation, an upper limit is 1000 nm and a lower limit is varied, and the temperature is calculated by using the equation of radiation of the blackbody from the heat radiation intensity in the measuring wavelength range. The calibration temperature is 1000° C., and the temperature of the measuring target is 600° C. From FIG. 13, it can be seen that when the lower limit of the wavelength for measuring heat radiation intensity is 900 nm (wavelength range is 100 nm), the error in the temperature is about 2° C., and that the error increases as the lower limit of the wavelength shortens (the wavelength range becomes larger). In order to make the error in the measured temperature at 600° C. be 10° C. or less, the lower limit of the wavelength is 800 nm (the wavelength range is 200 nm). The error in the measured temperature due to the first factor is determined by the measuring wavelength range of heat radiation intensity and the temperature of the measuring target. When only the first factor occurs, even when temperature control is performed based on measured temperatures, although there are errors in absolute values, stable control is possible and also the repeatability is good.
The second factor is ascribed to change in emissivity by forming the thin film on the substrate. When the selected wavelength range becomes wider, particularly when light absorption within the selected wavelength range is small in the thin film formed on the substrate, the influence of interference within the wavelength range for measuring heat radiation intensity becomes non-negligible as the thickness of the thin film increases. Specifically, when the heat radiation intensity distribution with respect to wavelength in the selected wavelength range changes more largely than when it is substantially constant, heat radiation intensity monitored in the selected wavelength range no longer faithfully reflects the heat radiation intensity at the center wavelength, resulting in that the measured heat radiation intensity containing a large error. The error due to the second factor depends on the thickness of the thin film formed on the substrate, and even when the temperature of the measuring target is constant, the apparent temperature would change. When, conversely, the temperature is controlled based on the measured temperature while the thin film is formed, the temperature of the measuring target actually changes, which is a big problem in control of temperature. Even if the temperature of the measuring target is the same, if the thickness of the thin film formed on the substrate is not the same, there is basically no repeatability in measured temperatures. To solve this problem, it is effective to decrease the wavelength range for measuring heat radiation intensity, so as to improve the precision of measurement of emissivity.
Thus, in the conventional radiation thermometer which corrects the emissivity, the precision of heat radiation intensity is lowered when the wavelength range for measuring heat radiation intensity is widened, and conversely when it is narrowed the signal strength of heat radiation intensity decreases, thus having problems that the measured temperature region becomes narrow, or that a long measurement time is needed, and so on.
An object to be achieved by the present invention is to provide a radiation thermometer and a thermometry method which are, without changing the wavelength range for measuring heat radiation intensity, capable of suppressing lowering the precision and the signal strength of heat radiation intensity so as to improve thermometry precision.